It is proposed to utilize biochemical and immunologic techniques in the development of radiostereo assays for vitamin D3 and its active metabolites, 25-hydroxycholecalciferol and 1, 25- dihydroxycholecalciferol. The assays will be utilized in in vivo and in vitro studies of the effects of age, nutrition, environment and hormonal status on the secretion, activation and distribution of the vitamin in man and animals.